1. Field of the Invention
The present invention relates to a method for compensating an offset temperature drift in a semiconductor strain gage sensor, using a semiconductor strain gage. The semiconductor strain gage sensor such as a pressure sensor, an acceleration sensor, an inclination sensor, and so forth used in an automobile, a machine tool, precision measurement equipment, an elevator, an aircraft, air conditioning equipment, and so forth, includes a bridge circuit formed by a semiconductor strain gage.
In such a strain gage sensor such using a semiconductor strain gage, the strain gage formed on a semiconductor substrate is constructed as a full bridge or a half bride. When a signal to be measured is inputted to the strain gage, resistances in a detecting portion in the strain gage are changed. The change of the resistances is converted into a change of a voltage to be outputted as a sensed signal.
In the sensor of this type using the strain gage, even when the input signal such as a pressure signal, acceleration signal, and so on to be measured is zero, an offset output voltage is provided. Since the offset voltage generally changes depending on the temperature, it is necessary to compensate the offset voltage with respect to the temperature. Hereinafter, the compensation of the offset voltage with respect to the temperature is referred to as a temperature compensation of the offset voltage.
2. Description of the Related Art
FIG. 1 is a circuit diagram showing an offset voltage temperature compensating circuit in a prior-art semiconductor strain gage sensor disclosed in Japanese Patent Publication (Kokai) No. 59-37417. In the figure, reference numeral 1 is a semiconductor strain gage sensor including elements, 2 is a compensating resistor having one end connected to either one of the output terminals Out1 and Out2 of the semiconductor strain gage sensor, 3 is an input power source connected between the input terminals In1 and In2 of the semiconductor strain gage sensor 1, and 4is a temperature-sensitive power source for generating a voltage which changes depending on the temperature. The temperature-sensitive power source 4 is connected between one of the input terminals of the semiconductor strain gage sensor 1 and the other terminal of the compensating resistor 2. Reference numeral 5 is a temperature-sensitive voltage generating circuit formed by the temperature-sensitive power source 4 and the compensating resistor 2. The temperature compensation of the offset voltage is effected by the compensating resistor 2 and the temperature-sensitive power source 4 connected in series between one of the output terminals and one of the input terminals of the semiconductor strain gage sensor 1.
Next, a description will be given for the conventional method to effect the temperature compensation of the offset voltage.
Let assume that the resistance of the strain gage semiconductor sensor is R.sub.g, and the resistance of the compensating resistor 2 is R.sub.c. Then, the resistance R.sub.g can be generally expressed as: EQU R.sub.g =R.sub.g0 (1+.beta..sub.1 T+.beta..sub.2 T.sup.2)
where Rs.sub.g0 is a strain gage resistance at the temperature of 25.degree. C.;
T=(t-25) : a temperature with respect to 25.degree. C.; PA1 .beta..sub.1 : a temperature coefficient of the primary order; and PA1 .beta..sub.2 : a temperature coefficient of the secondary order.
In addition, in the prior art, the model of output voltage V.sub.o of the temperature-sensitive power supply 4 is expressed only as: EQU V.sub.o =V.sub.S (1+.alpha.T)/2.
Then, the temperature compensating value V.sub.fc for compensating the offset voltage can be expressed as follows: ##EQU1## where n=R.sub.c /R.sub.g0, and V.sub.S is the voltage generated by the input power source 3. As can be seen from the above equation (1), the temperature compensating value V.sub.fc include in the bracket in the equation (1) a primary term and a secondary term with respect to the temperature T.
On the other hand, when the temperature-sensitive voltage generating circuit 5 is not connected to the semiconductor strain gage sensor 1, the output voltage of the semiconductor strain gage sensor 1 can be expressed as: EQU V.sub.out1 -V.sub.out2 =V.sub.f0 (1+A.sub.2 T+A.sub.2 T.sup.2)(2)
It is desired to cancel the primary order term and the secondary order term with respect to the temperature T in the above-equation (2) by the temperature compensating value V.sub.fc as expressed by the above equation (1).
Since the conventional temperature compensating value V.sub.fc is expressed as the above equation (1), however, the dependency of the secondary order term with respect to the temperature T in the bracket of the equation (1) on the temperature is determined only by the primary order temperature coefficient .beta..sub.1 of the strain gage. Therefore, even when the value a and n are so selected to cancel the primary order term V.sub.f0 A.sub.1 T in the offset value V.sub.f0 (A.sub.1 T+A.sub.2 T.sup.2) by the primary order term with respect to the temperature in the equation (1), a compensating error depending on the temperature coefficient .beta..sub.1 in the secondary term in the bracket of the equation (1) remains. On the other hand, if the value a and n are so selected to cancel the secondary order term V.sub.f0 A.sub.2 T.sup.2 in the offset value V.sub.f0 (A.sub.1 T+A.sub.2 T.sup.2) by the secondary order term with respect to the temperature in the equation (1), a compensating error depending on the temperature coefficient .beta..sub.1 in the secondary term in the bracket of the equation (1) also remains. Therefore, it is impossible to compensate both of the primary order component and the secondary order component in the temperature drift in the offset value to be compensated. Accordingly, in the prior art, there is a disadvantage in that it is difficult to compensate the offset value flexibly depending on the dependency of the temperature.